'All I'm trying to do is get the government to acknowledge the truth," says Jimmy Johnson, 63, once a model soldier and now a model prisoner. Johnson, currently in Frankland maximum-security prison in Durham, where for the last 25 years he has been serving his second life sentence for murder, is a man with a mission. "The prison system is awash with ex-servicemen," he says, "and unless the government, and in particular the Ministry of Justice, starts taking this problem seriously, things are going to get much much worse."

Johnson's particular concern is what he feels is the complacent attitude shown by the government not just to veterans who end up in prison, but to the related issue of those suffering from post traumatic stress disorder (PTSD) – many unknowingly. "This is an even bigger problem," he says, "and the way we are going with this war in Afghanistan it is going to get bigger. This needs to be addressed honestly and urgently."

Johnson was first jailed for life in 1974, less than a year after his honourable discharge from the Royal Tank Regiment following 10 years of "exemplary service". But when he left the army, he explains, his life quickly began to fall apart. Struggling to adjust, he began drinking heavily and experiencing mood swings which impacted badly on his relationship with his wife and two children. He managed to find some casual work "here and there", but without any real sense of purpose he drank more, and became more and more depressed. "I drank to dull the pain," he says, "but it never really went away."

One afternoon, four months after Johnson signed his army discharge papers, he reached his lowest point. He had tried to go for a drink in a social club, but it required an identity card to gain access. He did not have one. "They turned me away. 'You're not a soldier now,' they said. Nobody wanted to know me." He walked away.

A short while later, a van pulled up alongside him. It was driven by an acquaintance who worked as a security guard. He asked Johnson if he wanted a lift. "I had nothing planned," he says, "and just got in." As the van made towards the next stop on the driver's round – a factory – it slowed down to pass a group of children playing football. Suddenly there was a loud bang on the side of the van. Johnson thinks it was either the ball being kicked against it or a brick that had been thrown. Whatever it was, the sound triggered a horrific reaction in the former soldier. As soon as they arrived at the factory and alighted from the van, Johnson remembers grabbing hold of his acquaintance by the arm and neck and "running him". What he does not remember is then picking up a scaffold pole and beating the man to death with it.

Arrested the next day, Johnson had no explanation for what he had done. "But I knew that I had killed a man. That's why I pleaded guilty at my trial."

For that offence he spent a little over nine years in prison before being released on licence. While inside he expressed remorse for his actions but still could give no explanation. He received no therapy or counselling or indeed any kind of in-depth analysis to try to ascertain why he had behaved the way he did. His army discipline combined with regulated prison life made it easy for him to present himself as a "model prisoner".

These were the days when, other than perfunctory call-ups by prison staff, there was little focus on the motivations behind so-called offending behaviour. The closest Johnson came to any sense of rationalising what he had done was when, some years into that first life sentence, his father suggested during a visit that he was suffering from what many Americans returning from the Vietnam war had experienced. "My dad said to me, 'You've got what a lot of those yanks had after Vietnam,' meaning nightmares, flashbacks and other weird stuff. He meant PTSD but he didn't know that's what it was called and it was a long time later before I ever heard of it."

Johnson joined the army aged 17, and was first involved in action in Aden in the 1960s, where he demonstrated the leadership qualities that led to his promotion to corporal and tank commander. Later he served two tours of duty in Northern Ireland, during which he was involved in suppressing riots, controlling missile-throwing crowds, and clearing areas where bombs had been laid.

'The woman looked like a large rag doll smashed to pieces'

The incident that affected him most profoundly occurred while he was leading a mobile patrol of two Land Rovers and seven men through the centre of Lurgan in March 1972. A massive explosion a hundred yards in front of Johnson's vehicle brought the patrol to a halt. A bomb had been detonated in an underground toilet. Johnson saw people scattering through swirling clouds of thick black smoke. Within minutes he had cordoned off the area and mounted guards to provide cover against snipers in case it was a set-up.

Raised voices caught his attention. A man was screaming at a policeman and pointing at the toilet. "He was shouting, 'My wife's in there! My wife's in there!'" Johnson raced towards the man. "He was becoming hysterical and the policeman, looking terrified, was adamant the toilet was empty. But I could see by the man's face that he knew. He knew. I grabbed the policeman's torch and took two of my men down the gaping hole in the ground, climbing over rubble, slabs of brick and concrete and gushing water. In the thick smoke the torch was useless. We scrabbled around blindly. Then we heard a loudhailer above calling for us to get out fast as there was a car parked overhead with 500lb of explosives inside.

"I was about to give the order when one of my men found a woman's shoe. We dug frantically with our bare hands. Then I found her. She looked like a large rag doll smashed to pieces. All that was left of her clothing was a piece of rag around her neck; other parts of her had been blown off. Even her toes were missing. I covered her with my combat jacket and carried her out to an ambulance."

Johnson says he remembers shouting at the doctor in the back of the ambulance, "Save her!" Her says her face reminded him of his wife's face. "The doctor turned to me and very quietly said, 'Sorry son, but she's dead.' Outside the ambulance I started to shake and tremble and sweat like I had never sweated before. Someone led me into the back of a Land Rover where I sat and tried to smoke a cigarette. I heard women's voices and laughter, and then a woman leaned into the open back door of the vehicle and said, 'Don't worry, son, she was only a catholic." That night he was injected with a tranquilliser to help him sleep. "I never thought I'd be able to sleep again," he says.

Dealing with disturbances, which often developed into riots, was a routine duty for British soldiers in Northern Ireland during the troubles. They learned their techniques at "tin city" in Sennelager in Germany; a mock-up of streets and walkways, complete with shops and pubs that blared out Irish rebel songs. The facility had been specially built as a close-quarter combat range. There they learned how to fight pitched battles, with colleagues dressed as rioters hurling bricks and petrol bombs. They were conditioned to relish "aggro" and many were geared up and keen for the real thing once out on patrol on Northern Ireland's streets. "We carried personal weapons such as cut-down baseball bats, lead pipes and coshes, most of which had been handed down by troops that had been relieved," explains Johnson. "But towards the end of my time, and especially after the bombing of the woman, I was worn down by it."

The final straw for Johnson was an incident during a riot when he chased and caught a rioter who had attacked one of his men. "I began smashing him with the baton gun. I smashed him over and over regardless of the blood that was gushing from his head and oblivious to his screams. The next thing I remember is one of my men screaming at me, "Jimmy! He's had enough!" – that brought me back to my senses." Later, he says, his officers made light of the damage his out-of-control violence inflicted on the rioter.

For his actions in trying to save the woman's life in the underground toilet, Johnson received a "mentioned in despatches" – an award given in recognition of exceptional heroism or other noteworthy action. It was the highest award earned by somebody in his unit since the Korean war. But procedures to help soldiers deal with the aftermath of traumatic events relating to their service activities then were scant. There were few options available to help the soldiers wind down or de-stress after bombings or riots or incidents such as Johnson's beating of the rioter. Alcohol was the main medication. "We just went to the mess and drank ourselves out of it," says Johnson.

Finally he could face no more conflict and, despite the best efforts of his superior officers to persuade him otherwise, he bought himself out of the army in December 1973.

'Jimmy, you never laugh or smile like you used to'

When his father suggested during that prison visit that it was his traumatic army experiences that were behind his bouts of uncontrolled violence, Johnson was reluctant to make the connection. "I was a soldier. That was my job. It was only much later that I found out about PTSD. That's when it all started to make sense."

That there was something seriously wrong with Johnson is beyond dispute. Despite what she had had to cope with when her husband first came out of the army, and the shock of his arrest and conviction, his wife stood by him throughout his nine years in prison. But when he returned home following his release, it was clear that the problems he had been grappling with before were still there; indeed they had become worse.

"She said to me, 'Jimmy, you've changed. You never laugh or smile like you used to. I feel like I don't know you any more.' When she eventually left me I was lost. I started living like I had before, drinking, doing bits of casual work on building sites. I had a few ex-army mates I knocked around with, but nobody to talk to about my deepening anxieties. Even if I had I probably wouldn't have, as I still just thought it was me not coping for some reason."

Eighteen months after his release on licence, Johnson killed again. A week or so before it happened, he had read a newspaper report about an army friend who had been killed in Northern Ireland and his anxieties were exacerbated. While doing some building work for a man he had worked with before, he began to feel "agitated". He says he found himself staring out of a window, across the roofs and listening to the shrill sounds of children down below. "The next thing I knew I was hiding under a stairwell," Johnson says. "That's pretty much all I remember."

Sometime between staring out of the window and hiding under the stairs, he had picked up a lump hammer and beaten the man he was working for to death. Again he was arrested quickly and again he pleaded guilty at his trial. This time he was sentenced to life imprisonment with a recommendation from his judge that he serve a minimum term of 20 years, later raised by the home secretary to 30 years.

Yet again, little was made of Johnson's past military experiences, particularly those relating to his time in Northern Ireland. He knew he had killed another man and accepted his punishment. The only thing he did not know was what had triggered the two motiveless killings.

In 1985 he was transferred to Frankland prison, where he has remained ever since as high-security category A prisoner. It was in his early days at Frankland that he met and became friends with a fellow "cat A" also serving life – a man who had been an eminent medical doctor on the outside. He was interested to hear about Johnson's military experiences. "He said to me that he thought I was suffering from PTSD," Johnson says. "That was the first time I'd heard of it. He also gave me the contact details of a former colleague of his who specialised in it. So I wrote to him. This other doctor wrote back and explained that he had helped many ex-servicemen with various levels of trauma exposure, but I was the first he had heard of in prison. His letter was a godsend to me. Suddenly I had some understanding of what might have been going on."

Aly Renwick, author of Hidden Wounds: the Problems of Northern Ireland Veterans in Civvy Street and himself a veteran of that conflict, spent a number of years researching the history of PTSD-related criminal behaviour among British combat veterans. Renwick's conclusion in relation to Northern Ireland was that, "Probably more deaths and injuries have been inflicted on the civilian population in Britain by our returning soldiers than by IRA bombings."

Dr Claudia Herbert, a renowned PTSD expert who leads a team of specialists as director of the Oxford Stress and Trauma Centre, has worked with many ex-service personnel with experience of conflicts including Northern Ireland, the Falklands, Kosovo, Iraq and Afghanistan. Herbert's explanation of how PTSD affects those who have been traumatised is revelatory. "Military personnel are trained to deal with extreme situations," she says. "They may understand the theory of how to react, but when the action occurs the reality of what they are faced with can make them react very differently. What happens in a situation of extreme trauma is that the higher order processes, the cognitive processes, tend to shut down and the body tends to predominantly operate on autonomic functioning, which is geared solely towards survival."

In a life-or-death situation, explains Herbert, the event experienced gets stored in the memory system of the body, the mind and the brain. "Left unprocessed it remains a 'body memory'." The part of the brain that translates feeling into communicating the experience is often shut down during trauma, so that the trauma is stored but the person cannot say what has happened to them. "They may then experience it, relive it, through 'flashback'," Herbert says, "but can't actually talk about it."

Problems arise when military personnel leave the forces and try to fit back into normal life. "They are running around with a body full of stored memories and, because these have not been processed, they can be triggered by what otherwise seem like normal everyday events. A sudden noise or movement may trigger an extreme reaction over which they have no mental control. It is a survival mechanism. Such people who have been trained to kill are dangerous, perhaps not per se, but their bodies have been trained to be killing machines."

'Many people turn to alcohol or drugs to cope'

Not all who suffer from PTSD experience flashbacks, or become violent. "The symptoms are very uncomfortable," says Herbert. "They can lead to social withdrawal, emotional detachment, depression and difficulties in maintaining relationships. Many people turn to alcohol or drugs as a means of coping. But this can lead to even more problems." Herbert's big concern, like Johnson's, is that there is little official acknowledgment of the potential scale of the problem, given the fierceness of the many conflicts in which British forces have been involved over the last 40 years, and in particular the current conflict in Afghanistan where the close-quarter combat is the most sustained since the second world war. "The numbers actually affected might be far greater then we allow ourselves to believe," she says.

According to the Ministry of Defence, forces personnel now receive training to increase awareness of mental-health issues and stress management throughout their careers, and particularly prior to and after deployment. But is it enough? Increasing use is being made of Trauma Risk Management (TRiM), a model of peer-group mentoring and support in the aftermath of traumatic events, and the MoD's Medical Assessment Programme is also available to all veterans deployed on operations since 1982 who feel their mental health has been affected by their service experience.

Last month, health minister Mike O'Brien announced a joint government initiative with Combat Stress, the highly regarded veterans' mental health charity. The intention is that Combat Stress workers will work within NHS mental health trusts to ensure that veterans receive the treatment that they need in a "culturally acceptable" way.

But for some service personnel, doubts remain about the efficacy of the schemes. The culture in the forces, especially in the army, is to encourage high levels of controlled aggression and fearlessness. The conditioning is so powerful that it makes it almost impossible for anyone who feels troubled to step up and ask for help. One soldier, who does not want to be named, spoke about his experience during a 72-hour "decompression" period in a military base in Cyprus following several months on combat patrol in Afghanistan. "We spent three days getting legless and then they asked us if we had any problems," he said.

The Ministry of Justice claims just 2,500 prisoners were once in the forces. That figure some find hard to believe. Johnson for one is incredulous. "That's scandalous," he says. "I know from being in here for so long that there are thousands more than that."

While the Ministry of Justice admits that its research is incomplete, the discrepancy between its figure and the one produced by the National Association of Probation Officers (Napo) last year is huge. According to Harry Fletcher, assistant general secretary of Napo, this is because of flawed methodology. "The MoD database lacks many first names or dates of birth and it doesn't include reservists," he says. "And since it only goes back to 1979, anyone over the age of 50, say, would be screened out."

Last year, the probation service carried out probably the most accurate research so far, reaching the conclusion that there were around 20,000 people in the criminal justice system who were ex-forces; 8,500 of whom were in prison – around 10% of the prisoner population. The survey also showed that the number of veterans in prison had risen by 30% in the previous five years.

Johnson decided to appeal against his murder convictions in 1996. He intended to cite his PTSD as a mitigating factor and hoped that the convictions would be reduced to manslaughter on the grounds of diminished responsibility. A psychologist confirmed that he was being treated for PTSD and he was granted legal aid. Due to a number of factors, not least the struggle to find new evidence, time and money ran out and the appeal was never lodged.

Since then Johnson has spent his years in prison collating as much information as he could on PTSD and its effects on the mind. Since 2003 he has been self-publishing his widely respected Survival Guide for Veterans and Their Families, which is sent free to any ex-service personnel in prison who requests a copy. He is currently preparing a fresh application for appeal.

Johnson's story is a tragedy for his victims and their families, and for him and his family. But once he served his country well, as a leader of men into what were often the most unimaginably difficult situations. His breakdown following his discharge from the army should have been given serious attention a long time ago. As the conflict in Afghanistan moves towards possibly its most intense stage yet, Johnson's experiences should serve as a warning of the potential danger posed by undiagnosed hidden trauma. "After all," says Fletcher, "if these lads are good enough to be brought home in boxes at Wootton Bassett, they are good enough to get proper support and counselling before they end up in the criminal justice system."

The Survival Guide for Veterans and their Families by Jimmy Johnson and Hidden Wounds by Aly Renwick are both available from vetsinprison.org.uk